Chunk server: implement pending chunk reservation accounting to take into the account lazy space reservation that is performed only on the first write. This is especially important with write append as write append delays writes until it buffers enough data, or runs low on buffers, and small host files systems with only few GB of available space.
Chunk server: allow recursion of depth 1 in write appender low on buffers flush logic to handle possible need for partial buffer flash.
Meta server: fix dead code assignment. Stl container assignable requirement is the only theoretical reason not to get rid of the dead code.
Meta server: implement file storage tiers change with set replication rpc.
Meta server: implement Tree::changeFileReplication() parameters validation.
Chunk server: fix assertion failure in chunk allocation completion by adding missing reset of disk io smart pointer in the case when the request enqueue fails (due to out of requests).
Merge branch 'topic/disk_buffer_manager' into topic/net_io_threads